The present invention relates to a method for monitoring the bit error rate of a digital signal transmission according to the pseudo error rate technique.
In devices for receiving digital communication signals, apparatus is required to constantly monitor the bit error rate occuring during operation. Such apparatus may serve to actuate an alarm or to switch to a reserve channel when the bit error rate exceeds a given value.
One known method for monitoring the bit error rate is called the pseudo error rate measurement and is disclosed in the CCIR-Report 613-1 Doc. 9/1069-E, Jan. 16, 1978. In this method, the received digital signal on which interfering signals are superposed is regenerated twice: once in the optimum manner for the main signal path; and, in parallel therewith, in a secondary signal path in a manner which results in an intentionally degraded performance and increase in the bit error rate. The output sequences obtained by the different types of regeneration are compared with one another, and the number of disagreements, or noncoinciding signals at the two outputs over a certain time interval constitutes a measure for the bit error rate in the main signal path.
The intentional increase of the bit error rate in the secondary signal path can be obtained by modification of the decision regions of the regenerator; for this purpose one can shift either the thresholds of the decider or the scanning point in time can be shifted with respect to the optimum case.